Hops are primarily used in the brewing of beer. It is known that compounds derived from flowers of the female hop plant (Humulus lupulus L.) contribute a desirable bitter flavor to beer. This bitterness derives from the so-called α-acids, an homologous series of organic acids that are converted during the boiling of the brewer's wort into highly bitter, isomerized α-acids (iso-α-acids). Hops also contain an analogous series of β-acids. These substances, which have very low aqueous solubility, are of little value in brewing and are almost entirely eliminated from the wort by precipitation in the proteinaceous “trub” that forms during the boil. Many brewers now use extracts of hops which are convenient and much more stable that the traditional dried hops. Such products are made by extracting the hops with organic solvent (almost exclusively ethanol) or, more commonly, with carbon dioxide in either liquid or supercritical state. These extracts contain high contents of α-acids and β-acids, most of the remainder consisting of hop oils, waxes and uncharacterized resins. Typically, the α-acids content of a hop extract is in the range 35–65% by weight, that of the β-acids 15–40%. Hop processing companies have for many years also offered brewers a choice of more refined products that are prepared from the hop extracts by means of fractionation and chemical conversion, many of which may be added into the brewing process after fermentation of the wort. Such products include aqueous preparations of purified iso-α-acids, and their chemically reduced derivatives, especially tetrahydroiso-α-acids. In the course of preparation of these products, the hop processor will typically obtain a by-product fraction that comprises primarily a mixture of β-acids and hop oils, plus some minor components including waxes and small amounts of iso-α-acids. This fraction, commonly known as “β-Fraction” “Beta Aroma Extract” or “Base Extract” is often sold to brewers for addition to the wort kettle, where the hop oil component imparts aromatic flavors. However, it is also common to separate the oils from the β-acids, enabling a more potent “Aroma Extract” to be offered and releasing the β-acids for other uses.
In addition to being used for the purpose of contributing bitter and aromatic flavors to beer, hops are known to be useful to control bacterial growth during the brewing process. It has been demonstrated that the hop resin acids (α-acids, β-acids, iso-α-acids and chemically reduced iso-α-acids such as tetrahydroiso-α-acids) have anti-microbial activity and are especially active against Gram positive bacteria. Consequently, several uses for hop resin acids in food processing, cosmetic and pharmaceutical applications have been described. β-acids are generally considered to be particularly effective, natural agents. In WO 00/52212 it is noted that “certain hop acids exhibit anti-bacterial effects in sugar containing aqueous mediums. For example, European Patent Application No. 681 029 A2 discloses a process for inhibiting thermophilic micro-organisms in the presence of sucrose aqueous medium, in which a hop based product is added to a sucrose aqueous medium at temperatures between 50° C. and 80° C. And, U.S. Pat. No. 5,286,506 discloses a process of applying a solution containing beta acids to a solid food product to prevent growth of Listeria. According to Arch. Mikrobiol. 94 (1973), p. 159–171 beta acids exhibit the highest bacteriostatic effect in comparison to alpha acids and isoalpha acids; however, because of its poor solubility, certain concentrations of beta-acids cannot be exceeded.” Hop resin acids, especially β-acids, have also been claimed as effective antibacterial agents in food processing in US Publication No. 2002/0197366, U.S. Pat. No. 6,251,461 and U.S. Pat. No. 6,475,537, and have recently also been shown to have useful activity against algal growth in water systems (U.S. Pat. No. 6,379,720 and PCT Application No. WO 02/078450), protozoa (U.S. Pat. No. 6,352,726 and U.S. Pat. No. 6,423,317) and have been proposed as active agents against mastitis in cows, wherein the hop compound can be applied to the udders and teats of cows (US Patent Application Ser. No. 2003/0013773. The possibility to use hop acids in mouthwashes or toothpastes to suppress the activity of Streptococcus mutans and thus help to prevent caries has been described in U.S. Pat. No. 5,370,863. The general mechanism by which hop acids are believed to act against susceptible (Gram +ve) bacteria has been discussed by Simpson and Smith (Simpson, W. J., and Smith, A. R. W., 1992 in “Factors affecting antimicrobial activity of hop compounds and their derivatives”. The Journal of Applied Bacteriology 72 (4):327–334).
Because most agricultural products come from the field, bacterial contamination is unavoidable. Similarly, bacterial contamination of fresh meats is difficult to avoid.
Recently, the beet sugar industry has begun to use β-acids at a few ppm level as an agent for controlling bacterial growth in beet sugar processing. More particularly, the sugar industry has used β-acids in aqueous, alkaline solution as an additive during processing of sugar beets. However, since β-acids preparations are usually somewhat bitter (probably in the most part due to contamination with small amounts of the highly bitter iso-α-acids), they are used quite sparingly. WO 00/52212 teaches that an aqueous, alkaline solution of β-acids can be prepared at a concentration of about 10% by weight and that in this form the antibacterial activity is better than that of a similar quantity of β-acids applied as an emulsion or dissolved in organic solvent. However, a considerable practical disadvantage of such solutions is their tendency during storage to precipitate waxy or resinous matter that may foul pipelines and dosing pumps. The solutions must also be kept from freezing and so may require heating during transport and storage in cold times of the year. However, such solutions are relatively unstable. In contrast, we find that β-acids, when dissolved into propylene glycol in the form of their alkali metal salts, have chemical and physical stability over a wide temperature range and may be prepared at substantially and usefully higher concentrations. The use of propylene glycol as a solvent for many organic substances is well-known, and it has also been used as an aid to the solubilisation of isomerized and chemically reduced, isomerized α-acids. For example, Paul Todd, Jr. in U.S. Pat. No. 3,486,906 describes the preparation of iso-α-acids dissolved in their free acid form.